It knows when you try to pass off artificial sweeteners as the real thing.

Researchers at Yale University School of Medicine show that the pleasure we get from eating the sweet stuff comes from the amount of energy that we expect to gain from it. So the brain knows that low-calorie or no-calorie sweeteners are imposters when that burst of metabolism isn’t forthcoming. And that could explain why artificial sweetener fans may end up craving, and consuming, more calories over time than those who stick with sugar.

The researchers measured various behaviors as well as brain responses among a group of mice while they consumed either sugar or artificial sweeteners. When the mice satisfied their sweet tooth with the artificial sweeteners, they quickly lost interest in the sugar stand-ins when given the opportunity to choose, and desired the real stuff — especially when they were hungry and their sugar levels dropped. When sugar is metabolized to fuel the body’s cells, levels of dopamine hormone rise, contributing to feelings of satisfaction and reward; it’s the signal that the body’s energy needs are being met. But the mice consuming the artificial sweeteners somehow sensed that the sugar substitutes did not have the expected calories that their cells needed. The researchers believe people have similar reactions when they eat a low-calorie product when they’re hungry or tired.

In fact, previous work involving dieters who switched to artificial sweeteners or chose lower calorie options (like diet soda) suggested that people may even experience cognitive distortion and rationalize eating more later on. They convince themselves that since they saved on calories earlier, they can splurge on an ice cream sundae. The new study explains why, hinting that people may not be metabolically satisfied after consuming artificial sweeteners.

The findings don’t imply that eating more sugar is desirable. The researchers say that instead, people who want to reduce their sugar intake but avoid the body’s deprivation effect should compromise, and combine artificial sweeteners with a little bit of real sugar.